Prominence means the state of being important, hence Keyword Prominence refers to how prominent your keywords are within key elements of your web page, importantly how close your keyword or keyword phrase is placed to your title tag, and header tags (H1, H2 ,H3, meta description, Strong etc), therefore to create higher prominence you need to place your keyword near or within these tags.

For example:
My Keyword is “FindNerd: Social Network for Developers”

Hence This keyword should be included before the other keywords and should be placed on top with more priority.

Hence, Keyword Prominence play an important role for improving the keyword rank of targeted keyword on search engines. So you should always try to define the prominent use and place of your targeted keyword.

For Ex: there are 3 Keywords K1, K2 and K3 in which K1 is main primary keyword and K2, K3 are secondary. So you should always try to make good use of K1 with synonyms and also try to place it ahead of K2 and K3.