Is a string og letters and numbers representing stocks, mutual funds, options and other instruments. It is a "code name" used in connection with buying and selling on marketplaces all over the world.
Each instrument/stock thus has its own unique "ticker"/code that tells which instrument it is. And you can use this "ticker" to search for the spesific instrument on to the stock brokers webpages.
An example might be the company Tesla, Inc. This stock has the ticker; TSLA. You can then use this ticker instead of the whole name of the stock, so look for it at a stock broker.