The speaker is thinking about a grave that has disappeared into a garden, overgrown or blended in with the plants around it. The poem never says whose grave it is, but the word "her" and the tenderness of the tone suggest someone the speaker loved or knew closely. That lost grave has become "a serviceable friend," a plain, practical phrase that makes it sound useful rather than grand, something that quietly does its job.
The next line is the most puzzling. Calling it "a false teaching which eases pain" suggests the speaker knows that forgetting, or imagining the dead as part of the garden, isn't strictly true, yet it comforts anyway. The poem seems to admit that some consoling ideas are fictions we lean on.
The last line, "water in my eyes," is understated. The speaker doesn't say they are crying; the tears just appear. The mood is quiet, restrained and gently sad, with grief held in check but still leaking through.