Interleaving (aka mixed practice or varied practice) is a learning strategy that involves mixing different topics, ideas, or forms of practice to improve outcomes as well as overall productivity. Its opposite and less effective strategy is blocking (or block study or specific practice) which focuses instead on working on limited topics or single forms of practice at the same time.
This may be one of the values of of the Say Something In Welsh method which interleaves various new nouns and verbs as well as verb tenses in focused practice.
Compare this with the block form which would instead focus on lists of nouns in a single session and then at a later time lists of verbs in a more rote fashion. Integrating things together in a broader variety requires more work, but is also much more productive in the long run.