If one piece of land or a building abuts another, it sits right next to it and touches it along a side or an end. Surveyors, lawyers, and people who write about houses and roads use this; you will almost never hear it at the dinner table. A garden may abut a lane; a new wing may abut an old wall. The two things are neighbours along a line — they are not mixed into one, and they are not merely facing each other across a gap. Though actually, writers sometimes stretch it a little: “the park abuts the river” can mean they meet along the bank. The word is formal, precise, and a bit old-fashioned in speech. It has nothing to do with about, even though the spelling sits close.