Tracking is what your guns / launchers do automatically. They attempt to track (keep the barrel pointed at) the target as it moves through space. A big slow ship sitting still is easiest to track. A small fast ship moving around your ship is hardest to track. If you are at a decent range and the target is coming straight towards you tracking is a lot easier (you can pull range and make them chase you in a straight line by using the ‘Keep At’ option).
Tracking speed values are listed on guns and can be seen if you hover over the gun icons in space. I have some large guns on big ships with dreadful tracking speeds like 4. I have small guns on little ships with good tracking speeds like 400.
Generally using a big gun to hit a small target is like trying to use this:
to shoot this:
This is why size isn’t everything in EVE and small ships can defeat much larger ones.
There are all sorts of ways to enhance your tracking speed, including skills, modules and implants.
Targetting is locking enemy ships so that your guns will fire at them. If you don’t have a target locked you can’t fire your guns. You can lock multiple targets depending on how many your ship allows (ships have limits) and how many levels you have in the Target Management skill.
Importantly, locked and selected are 2 different things. You can lock many targets at once but have only one selected, and you can select one target after another and fire a different gun at each. You can change which one is selected (current active target) by clicking on the locked ship circle icons.
For example, you could target / lock 5 enemy ships, click one (now selected / active target) and press F1 to fire gun one at that enemy, then click another locked target (now selected / active target) and press F2 to fire gun two at that enemy.
You can tell which enemy is the currently selected / active target as its circle icon will have arrows spinning around it.
Commonly people group all guns (there is a link symbol, looks like a sideways 8, just left of the gun icons when in space, note - all guns need to be the same to link up) under one button (say F1) and fire them all at one target to dispatch that target as quickly as possible.
The logic behind focusing fire on one enemy at a time is that 1 less enemy when you are fighting 5 enemies means they now only do 4x the damage to you, while 5 partly damaged enemies still do 5x full damage against you. This isn’t always needed, if you can take the damage you can do as you please, and if your damage is high enough that you one shot everything with one gun anyway, hitting each with one gun actually becomes the better strategy.
Hope that helps