Affinity is the key feature of Marshal Monsters, represented by a silver star inside of a royal blue circle, akin to Levels and Ranks. The amount of Affinity on a Marshal Monster, represented by a line of such stars centered on the card between the name and the card artwork, represents the total number of Levels, Ranks, Qualities, Link Arrows, etc. on the field that a player must equal or surpass to Marshal Summon their Marshal Monster. Each of these units is treated as equal to the others in this regard: a Link 2 Link Monster is worth as much towards Affinity as a Rank 2 Xyz Monster or a Level 2 Effect Monster. Therefore, it is more advantageous to perform a Marshal Summon with Affinity gained from the raw materials before going for the larger monsters with either less Affinity or more concentrated amounts of it.
However, the quantity of Affinity is not the only deciding factor in a Marshal Summon, but also what that Marshal Monster has Affinity FOR. Affinity only counts towards a Marshal Monster if that Marshal Monster lists. For example, "Swordsman of Legend" is an Affinity 6 monster, meaning " " and " ", both Level 3 Warriors, would count as 6 Affinity, however, since both are Warrior, they count towards "Swordsman of Legend". However, if you instead had " " and " ", then there is only 3 Affinity towards Warriors from "Marauding Captain", while there is 4 Affinity from the Level 4 "Marauding Squad" towards Beast-Warriors, so "Swordsman of Legend" cannot be Marshal Summoned.