A meditation anchor is something you can draw your attention back to when you catch yourself starting to get lost in thoughts.
Anchors are ideally derived from observing your body or one of your five senses.
my favorite 2 choices are breath and sound:
Don’t force your breathing. Just let it happen naturally observe it and feel the resulting sensations/movements of your body (the rising of your belly or the air going in and out your nose)
E.g. bird song, rain, waves, or flowing water. The free meditation app InsightTimer offers a large number of options to chose from.