Literacy - the ability to understand, evaluate, use, and engage with written texts - is the foundational skill from which much else flows. Almost 20% of American adults cannot read at the 5th grade level. That means a fifth of Americans 18 and over are functionally illiterate (or barely literate if they're hovering around the 4th-5th grade level). Another third of Americans read at middle school level (6th-8th grade). That means they have basic literacy skills. Basic literacy skills can only take you so far - not so far as a four-year college degree.
I administered reading comprehension tests to adults for decades. One take-away: adult clients who tested less than 5th grade level in reading rarely advanced beyond that level - even with intensive one-on-one tutoring for up to a year. Their job options were limited. The functionally illiterate/barely literate men were luckier than the women: at least they could do decent-paying physical labor. The women were mostly consigned to low-paying jobs, like personal care assistant. This is depressing, but thank God for tax credits, food stamps, and housing assistance (among other safety net programs) to push many of them above the poverty line, if only slightly.
What I found with many adult clients reading at the "basic" level (roughly, between 6th and 8th grade level) was that with intensive basic skills training, they could improve their skills even further. If they didn't have a GED, they could get one with sufficient preparation. If they already had a GED or high school diploma, they could complete vocational training or a community college program. And then reaching the middle class was within the realm of possibility. Not immediately, but (as they say) the trajectory had been potentiated.
References:
Literacy Project Foundation: Staggering Illiteracy Statistics. http://literacyprojectfoundation.org/community/statistics/ Accessed 3/26/2017