It comes from the last words spoken by St. David, the patron Saint of Wales, whose feast day is the 1st March.
The whole sentence he was supposed to have said is “Be joyful, keep the faith and do the little things that you have heard and seen me do”.
In the literal sense, it means if you keep doing things by putting one foot in front of the other, they will eventually make up to bigger and better things in the long run.
Additional information can be found on the St. David’s Day page on the Visit Wales Official Website.
“Byddwch lawen a chadwch eich ffydd a'ch cred, a gwnewch y pethau bychain a welsoch ac a glywsoch gennyf i.”
English Translation
“Be joyful, keep your faith and your creed, and do the little things that you have seen me do and heard about.”