Aegis is a Special Save, with a value indicated in brackets (X). If X is given as a modifier and with a maximum value (e.g. +2), the model gains this as a modifier to all its Aegis Special Save rolls, which cannot be increased to rolls better than 3+. If the model doesn’t have Aegis, it instead gains a Aegis(7 X+). For example, a model without Aegis that gains Aegis(+2) will gain Aegis(5+).
Note that Aegis Saves cannot be taken against attacks with the Divine Attacks Attack Attribute.