結ぶ means to tie, bind, or conclude. It is used for tying physical things like shoelaces, as well as for binding a contract or for a plant to bear fruit. It implies bringing things together to create a new connection or result.
That is how Fuguro works. It brings 結ぶ back for review right before you forget it, each time a little later, until it sticks for good. You hear it spoken aloud, and you meet it again in stories you can already read. The first five levels are free.