Typically, ants aren't destructive. They are mainly a nuisance. The ants you see inside your home are called worker ants. They are searching for food. Once they find it, they'll invite more ants in by leaving a pheromone trail for them to follow. If they are entering through a crack in concrete, and the opening is left unchecked, you can soon have an ant invasion. That's why it's a good idea to seal those cracks up. It's not a difficult task or expensive to do.
Clean any loose concrete, dirt and debris out of the crack with a wire brush first before proceeding to fill in cracks to keep ants from entering.
Pull the plunger on the caulking gun all the way back. Insert the caulking tube into the barrel so the nozzle is at the front. Snip off the very end of the nozzle at an angle to make the caulking ooze out in a small stream. Insert a wire into the nozzle end and push it down to puncture the caulking tube. Remove the wire.
Position the nozzle of the caulking gun at the beginning of the crack, with the shorter angled side down. Slowly squeeze the trigger. When the caulking starts to ooze out of the nozzle, slowly move the gun down the crack from the stream, filling it in as you go.
Push the caulking into the crack once it's completely filled. Scrape the blade of a putty knife across the crack from one end to the other. This will push the caulking down into the crack, plus remove any excess. Wipe the blade off with a rag to clean it. Squeeze more caulking into the crack and level it if the caulking you applied didn't make the crack level with the surrounding surface.