mat, repeatedly
荐 centers on the idea of something placed or occurring in succession: a mat laid down, or an action done repeatedly. The two senses connect through the image of things following one after another.
荐 combines 艹 (grass) with 存, which likely contributes the sound and the idea of existing or being placed. The character originally referred to a grass mat, with the 'repeatedly' sense developing from the idea of things laid one after another.
Grass 艹 placed over 存 (exist) suggests a grass mat that exists underfoot, or grass that keeps coming back repeatedly.
For セン, imagine a mat made of a thousand (千, せん) blades of grass woven together repeatedly: 千 -> セン, and the mat is laid down again and again.