To add on what Machine Medic said: The goal is to make it so no enemy player is on the map OR able to spawn on the map.
There are a few basic strategies you can do to accomplish this...
For Aliens:
1. Destroy the enemy reactor will make it so all buildings will stop functioning. Humans are basically screwed here unless they are able to spawn a ckit a build a new one.
2. Destroy the enemy armory(s) will make it so humans cannot rearm or switch gear. In smaller games it basically means a human needs to die to spawn with a ckit and build a new one. During sudden death, this dooms the human team as they can not longer get any armor or weapons.
3. Destroy the enemy medistation(s) and the human team cannot easily heal (I think you can still get medikit from armory though) During sudden death this ensures you can easily get resources to continue an attack as humans will constantly die and respawn.
4. Destroy enemy telenodes directly. This is very effective during sudden death.
5. Destroy enemy turrets and other defenses to gain access to the above 4.
For Humans:
1. Destroy the enemy overmind will make it so that enemies cannot evolve. This gives you a steady flow of income to continue an attack as it forces the enemy to spawn a granger and use low hp pitins to defend.
2. Destroy the enemy booster will make it so that the aliens can't heal as fast and can no longer poison you. This is generally priority one for me, especially if the alien base is far away from the human base.
3. Destroy enemy defenses, in particular trappers and other slowing structures to give you access to overmind and spawns.
4. Destroy eggs directly, can be effective in sudden death if you can find them all.
This just scratching the surface for the game, as there is a lot of other strategies that may come into play especially when it comes to coordinating with your team. At high levels of play feint operations, rushes together as a team, and protecting your builder to create a forward base are all important factors that can lead your team to victory.