A chokepoint is an area of a map that is hard to pass and easily defensible. Most are narrow, limiting the rate at which units can move through them, with high hills or deep water on either side. Chokepoints are natural locations for defensive lines, and maps that are very chokepoint-heavy tend to encourage porcing.
Common practice is for both players to expand through several chokepoints until they meet the opponent's forces, then fortify the border chokepoints and use artillery to pound away at the enemy's fortifications. Pushing too far can leave you open to artillery fire from unassailable enemy positions, while not pushing far enough can leave too much metal in the hands of your enemy. Once a player's started producing level 2 units, assaults on chokepoints tend to become feasible again, as most fixed defences capable of repelling a focus assault by L2 units are quite expensive.
Most mods offer ways around chokepoints. In OTA-based mods, there are a number of options. Aircraft are the simplest, and can just fly over any chokepoint. This can work well, but a sensible opponent will have heavy AA protection for his base, making air assaults troublesome. Chokepoints bordered by hills can sometimes be passed by KBots, but often the hills are too steep even for them to climb. In this case, truly all-terrain units like the Spider become invaluable. Even if no other all-terrain units are available, Spiders can swarm over the hills and paralyze the defences before a proper assault. Against chokepoints bordered by deep water, hovercraft or amphibious assaults can give one's enemy a nasty surprise.