Ableism is a set of beliefs and practices that devalue people with developmental, emotional, physical or psychiatric disabilities. It occurs on a systemic level with practices like building inaccessible buildings or paying people with disabilities subminimum wage. This systemic ableism is reinforced on an individual level by not noticing injustices and working for change, and continuing to treat disabled people as "other".
On a person-to-person level, you should follow these three general rules when interacting with disabled people:
1. Always ask if you can help, rather than automatically assuming that someone needs assistance and jumping in. This is important when interacting with any patron, as everyone has different things they may or may not need assistance with which may not be immediately apparent. When working with people with readily visible disabilities, keep in mind that even if they may appear to you to be struggling, they might be doing fine and not want any outside assistance. On the flip side, not all disabilities are visible, and asking if someone would like any assistance gives them an opportunity to let you know about any specific requests they may have.
2. Address disabled patrons with the same respect you would give anyone else. Speak to them, not over them. If you're not sure what terminology to use around someone, and it's 100% necessary for you to address their disability in the context of the conversation, politely ask. People have different preferences for which terms they're comfortable with others using. Currently it's widely accepted to use person-first language (i.e. "people with disabilities" instead of the identity-first "disabled people"), since this type of wording is intended to combat the dehumanization that often accompanies the way people talk about people with disabilities. However, some others prefer identity-first language because they feel it helps normalize their disability by removing the assumption person-first language often carries that being disabled is a bad thing others need to look beyond in order to treat them like a person. In most situations, though, the most polite way to address someone is as you would anyone else.
3. Stay aware of personal space. Some patrons use mobility aids like wheelchairs or walkers, or may have a service animal. Always ask permission before touching or interacting with any of these. Service animals in particular should not be distracted from their job and although they may be cute and fluffy, this is not an invitation to pet them.
http://www.stopableism.org/p/what-is-ableism.html
Dolmage, Jay Timothy. Academic Ableism: Disability and Higher Education. University of Michigan Press, 2017. JSTOR, www.jstor.org/stable/10.3998/mpub.9708722
https://autisticadvocacy.org/about-asan/identity-first-language/