Yes it is a sucker.
If your tomato is a bush type, leave it, the suckers end in trusses of tomatoes.
If it is a vine type, you have a choice:
1) It is more work to remove it, because you'll have more tomato plants to fill your garden, (they can be placed much closer together) they will need repeated pruning, but your crop will start earlier, be heavier overall & better quality.
2) You can leave the suckers & it'll be far less work, & you'll get more tomatoes per vine, but the harvest will start later, & the tomatoes, although much better than shop-bought, will not be quite as good as pruned vines.
That is for if you are growing in N America.
I prune; we have little choice in the UK, as tomatoes in certain parts are marginal & our light is not nearly as good.