Can someone explain Enemy "Levels"vs. Tenno "Levels"

Can someone explain Enemy "Levels" versus our Tenno "Ranks" and if our actual "Level" comes into play during a mission?

If I jump into a mission on Earth it says Lvl 15-22 Enemy Level. I always assumed that meant I should have a Warframe and at least one weap near that number in RANK. Until I jumped over to a new system and they were like 30-40+.. Does our actual "Level" come into play within the mission? (i.e: higher damage output versus a lower lvl mob)

In part with the last question, does a newly bought/made weapon say rank 1 have a lower damage output or is JUST the absence of mod slots?