Firstly: The Meaning of the Restrictions of Ihram
Linguistically, restrictions are what is prohibited, itself being a synonym for haram.
Technically, restrictions of ihram are prohibited acts that one in ihram must avoid due to being in ihram and engaging in rites.
Secondly: The Definition of Expiation
Linguistically, expiation is to offer something in place of another in order to protect it. From this is the expiation (ransom) of a prisoner and freeing them with wealth.
Technically, expiation is what is required upon engaging in a prohibited act or omitting a wajib act. It is called fidyah (ransom/redemption) due to being mentioned in the Word of Allah: “…a ransom of fasting or almsgiving or offering…” (Al-Baqarah, 196)
Thirdly: The Number of Restrictions of Ihram
There are seven restrictions which apply to men and women:
Cutting the hair.
Clipping the nails.
Entering into a marriage contract.
Two restrictions are specific to men:
Wearing sewn garments.
Covering the head.
Two restrictions are specific to women:
The face-veil (niqab).
Wearing gloves.  Wearing gloves is prohibited for men as well, and is included under wearing sewn garments, which is prohibited for men in ihram.
Fourthly: Categories of Restrictions with Respect to Expiation
Restrictions of ihram are of four categories with respect to expiation:
Acts whose expiation is for suffering (the expiation of suffering is one of: blood, feeding the needy, or fasting).
Acts whose expiation is recompense with the like of the offense: hunting.
Acts with no expiation due: marriage contracts.
Acts with the maximum expiation: sexual intercourse.