Recaulking Bathtub for bathroom

[Paiea Palomanu]

So I have just brought this duplex, my first DIY project is recaulking this bath tub. Towards the middle the gap between the tub and the Tile was getting significantly bigger then a quarter inch. I was trying to figure out what to fill in the gap with before caulking ? I read a foam backer rod could help I just wanted to hear from people with more experience than me lol.