The summoners level determines the level your cards will have in battle.
For example. You might have a level 5 card. But if the summoner is a level 2, then it won’t be able to accommodate a level 5 so you will be forced to use the lower level the summoner accommodates.
The image above shows the level the summoner can accomodate the different level of cards.
See the image above. A level one Alric can only accomodate a level 1 rare, level 1 common, level 1 legendary, level 1 epic.
Level 2 Alric accommodates level 3 common, level 2 rare etc.
Do you get it now?