The dictionaries listed here are explanatory dictionaries, which means that the meaning of each word is explained in English. Some dictionaries contain a thesaurus, which is a dictionary of synonyms and possibly antonyms.
I consider Cambridge Free English Dictionary to be the best online dictionary. This dictionary contains a classical dictionary, a translator, a grammar interpretation and a thesaurus. Sample sentences are given for each vocabulary word. For important vocabulary, the language level is indicated, e.g. A1, A2, B1.
The Merriam-Webster Dictionary and Thesaurus is a classical dictionary and also includes a thesaurus. After registration you can save the selected words. Unfortunately, the site contains many inappropriately placed advertisements.
The dictionary Lexico is created by Oxford, but it contains too many advertisements that make the dictionary very difficult to use. There are plenty of sample sentences available. These pages contain explanations and examples of grammar.
This site Macmillan Dictionary contains a dictionary, collocations, phrases and a thesaurus. The videos explain vocabulary and give examples of vocabulary use.