Foundation; the basic or fundamental part of something—basic knowledge or skills that form the basis for further study or practice.
日本語の基礎をしっかり学んでから応用に進もう。
Learn the basics of Japanese well before moving on to applied aspects.
基礎練習を怠ると上達は遅くなる。
If you neglect basic practice, progress will be slow.
建物の基礎がしっかりしているので安心だ。
I'm relieved because the building's foundation is solid.