Each of your players is capable of improving, provided they are trained correctly. Similarly, if you neglect your players or train them excessively or in a counter-productive manner, they'll become less effective.
This training screen enables you to quickly assess the effects of training on your players and to modify their workouts, either to counter weaknesses or enhance their strong points. To simplify your management of training, the screen is split into three segments: the main schedules option in the top left, schedule details in the top right and, below them, the training effects screen. You can also see the effects of training on a player's training overview which displays a simple graph of how a player has responded to your tutelage.

Main Schedules
The main schedules window shows how many people are undertaking each schedule. To begin with, your players are working to preset routines which may not get the most out of their abilities. If you highlight one of the schedules a breakdown of that programme appears in the top right window, while the players on that programme are shown in the bottom window. To create a new schedule click the Schedule drop-down and select a Full-time or Youth option. What you ask your players to do in this routine is entirely your call, but you should make sure you give them a rounded experience to keep them developing in a positive way in overall terms, even if you want to emphasise, say, strength training to improve a players' stamina and strength.
Note that hovering over a training discipline name will show which player stats that exercise is likely to improve.

Schedule Details
Once you've created a new schedule you can use the schedule details (top right window) to alter the program in one of two ways.
Firstly, you can alter the intensity of each of the training disciplines undertaken with the slider bars. For outfield players the goalkeeping slider should be set to zero at the far left of the slider. You can fine-tune these sliders as much as you wish, but when you reach a point when it would be physically impossible to do any more (as indicated by the workload slider) you will not to be able to increase any of the disciplines.
The second way to alter the intensity of training is to simply adjust the workload slider alone - this will keep the same proportionate training regime but will save you having to tweak every slider down.
If you wish to fine-tune the training for individual players you can also use the Player Training option accessed through the player's own screen.
Note: A very heavy workload with lots of high intensity activities is sure to leave your squad vulnerable to injuries. It may have the short-term benefit of boosting stats more quickly, but in the long run you will not be able to avoid having to deal with players on the treatment table. Keep an eye on your players' conditioning as low fitness can often be the result of over exertion on the training pitch.

Training Effects
All players are unique and respond differently to each training schedule, so it's important to use this screen to carefully monitor the progress of your squad. It may sometimes pay to have a specific training schedule for individual players, though it's possible to tweak player regimes in terms of intensity to try to get the most out of them.
One of the key advantages of managing a club with good resources is that you can afford to bring in plenty of coaches to take control of various training schedules. You can use them to give your players the benefit of small group tuition with trainers specialising in the field you are dealing with.
Note: If you wish to suspend training for certain periods, such as in the closed season, you can do so by altering the relevant setting on the Manager Options screen.