I'm reiterating what FLC said, but
Skirmisher: Pokes at the enemy team and draws them out, so that the
Slayer: Comes in and splats the distracted/weakened opponents.
A good skirmisher feels like a constant thorn in the enemies side, not letting them focus on the objective for too long and constantly taking some of their attention and resources.
A good slayer feels like they came out of nowhere, getting multiple splats and map control before disappearing again.
The slayer can't do their job until the skirmisher draws the enemy out, and there's no point to the skirmisher if the slayer isn't there to finish off the opponents. The skirmisher and slayer will be switching roles, as you said, but one weapon in the pair will clearly be better at one job than the other. I don't know what weapon(s) you play, but some weapons are commonly accepted to be firmly in one role or another, while others are a bit more ambiguous.
DISCLAIMER: This response and flc's essay really only apply to comp teams and not solo queue, as you can't pick your teams comp there. Take all this with a grain of salt, as I have only read essays and analyses, and studied comp matches. I haven't played a whole lot of comp myself, but I understand this concept enough to explain it. Don't take this as gospel, just as guidelines for teams to make comps.