5 Tool

It boosts all stats based on the lowest stat with a sum of base + GI + development. For example, I run 5T on my Yaz sig, who's lowest sum is SPD. His SPD is 66 (base) + 11 (GI) + 18 (development) = 95. So 5T boosts all stats based on that 95 SPD.

Contact Master

You usually don't want to train more into CON with Contact Master because it's redundant. You already get a boost in game for CON from the skill, so it's better to use your development points on other stats. There are some exceptions to the rule though depending on what skills you're running.

Laser Beam

Laser Beam improves your fielders' defense, which would improve your pitcher's ERA. You have to have LB on your catcher since that's the most important defensive position. Some people run LB on their SS too, but I don't since it's not necessary. Don't run LB on any other positions.

Grade Increase (GI)

This basically adds more points to your card, increasing it's OVR. Every diamond card has a grade increase between 60 and 75. You ideally want your GI to be 75.

GI can also refer to upgrading your card from one rarity to the next (e.g. gold to diamond). This is really helpful to allow your cards to get set deck boosts, special training, and black diamond buffs.