The amount of opportunity available to a given person can vary greatly. For example, one person could have college paid for by parents, another could work to pay for college, and another could not have the opportunity to work to pay for college (e.g. perhaps they are taking care of younger siblings or sick family members).
Available effort could likewise vary. For example, someone with a disability might have much less effort available or their effort may be significantly less efficient. Someone who has to expend a great deal of effort on other matters (e.g. providing for basic needs, self protection, caring for others, etc.) may have less effort to expend towards things like career advancement or education.
The argument as I see it is that a person expending a roughly average amount of effort should be able to achieve a reasonable quality of life with the luck and opportunities that are available to them. (There will of course always be some variance in the effort required and the outcome received, but that's fine as long as it stays within a reasonable distance of this goal.)
The increasing anger over inequality seems to me to be because people are being asked to give above average effort (e.g. working multiple or demeaning jobs) in exchange for a low quality of life (e.g. difficulty affording healthcare, housing, retirement, raising a family, etc.).