The role of a support is more or less helping the team stay coordinated and watching for flankers as you will generally be in the back to mid lines of the fight so you'll have a greater view of the battle going on so it's easier for you to make call outs and letting your team know when they are over extending or too spread out
Main healer refers to the healer with the more consistent healing output like Mercy although you do want to be damage boosting more than you heal as her because that's where the real value on her character lies
Off healer is someone like Moira as she has her heal on a recharge and she has a fair amount of fighting capability with her biotic orb and fade (orb to engage, fade to run away)
If you're just sitting there spamming heal then you're effectively making it a 4v5 unless the dps can really pick up slack because you're completely negating half your kit and losing a lot more value and if your tank is a zarya for example unless she is getting completely stomped in the fight you should only heal her just above to where the shield starts coming back then engage the fight with her to add on to her damage
I'm not the best baptise so I can't really give you much more than these 2 patterns you should get comfortable with [heal,heal,shoot] for a defensive team fight [shoot,shoot,heal] for a push
Some of my favourite match ups to play
Mercy/Moira Bap/Kiriko Kiriko/Lucio Moira/Kiriko Ana/Lucio Brig/Zen Zen/Bap