I know nothing about home improvement, if isn't already obvious. All I can say is that I just moved into this townhouse and I just figured out there is this gap where ants seem to migrate in. I will just post a picture of issue because I don't know how to explain it very well. Any help would be great!

 

Welcome to the forums! Either your threshold needs adjusting upward, or in the event it is not adjustable, the door bottom sweep needs replacing. The door will need to be removed from the hinges and the old sweep removed and replaced with one identical. Many styles can be found at the box stores, and some universal types are available, as well. Frost King E/O 1 in. x 36 in. Brown PVC Door Bottom Replacement for Stanley Steel Doors-UDS36 at The Home Depot

 

Welcome to the forums Teresa!

It doesn't look like you have an adjustable threshold. You might be able to install a lattice strip [1/4"x2" wood] onto the threshold under the door. There are also weatherstripping that can be attached to the door that will hang down further..... but I'm just a painter, the carpenters should be along shortly with more [probably better] suggestions

 

I think the door is hung wrong. There is a gap that grows from left to right. The door appears racked with top right pulled in too far.

Can you post pictures that show the gap between the door and the frame? In other words, take a shot of the complete door.

 

Open the door and take a picture of the threshold. Then we will be able to tell if the threshold is adjustable or not.

 

Thank you for all the information!