Here's the simple explanation:
Countable nouns refer to things that can be counted. There is a singular form and a plural form that usually adds an -s or -es. For example:
1 chair - 2 chairs
1 potato - 2 potatoes
1 lion - 2 lions
1 clown - 2 clowns
Uncountable nouns are usually substances (ie: water), qualities (ie: patience), feelings (ie: love) or concepts (ie: experience). There are also some things that we usually don't count, but refer to generally (ie: luggage, machinery). These types of words do not have a plural form.
So how do we know if "luggage" means one bag or two? We use quantifiers to give us that information:
I have some luggage over here.
I have three pieces of luggage.
I only have a little bit of luggage.
I only have one item of luggage.
I have a lot of luggage.
Here is a list of the most common uncountable nouns that I often hear people add an 's' to. Make it your goal this week to use 3 of these words in their correct uncountable form.
advice: He gave me some great advice.
information: The speaker provided so much new information.
work/paperwork: I have tons of paperwork.
baggage/luggage: The airline allows two pieces of luggage.
feedback: He gave me some great feedback after my presentation.
furniture: We bought some new furniture over the weekend.
machinery: The factory is filled with complicated machinery.
equipment: They just renovated the fitness center and bought lots of new equipment.
staff: Make sure to tell your staff about the policy changes.