In short: consumer rights are the legal guarantees you have as a buyer of goods and services — to withdraw from an online purchase, to receive goods without defects, to know the final price and to complain.
Simply put: Four rights across the EU: 14 days to withdraw from a distance purchase without giving a reason; 2 years of seller liability for faulty goods; 14 days to withdraw from a consumer credit, including "buy now, pay later"; a refund of a payment you didn't authorise if you report it in time (within 13 months). The trader must ask for your explicit consent to any extra payment.
Example: Iva receives shoes on the 3rd. Until the 17th she can withdraw without a reason and must get her money back within 14 days of withdrawing.
Watch out: the right of withdrawal does not cover everything — for example made-to-order or perishable goods.
Example: Bulgaria — Consumer Protection Act: 14 days to withdraw (Article 50), refund within 14 days (Article 54); complaints — CPC.
Check in your country — your national consumer protection authority and the Your Europe portal.
Source: Directive 2011/83/EU: https://eur-lex.europa.eu/eli/dir/2011/83/oj · Your Europe — returns: https://europa.eu/youreurope/citizens/consumers/shopping/returns/index_en.htm