Bitcoin is a digital currency (cryptocurrency). A form of payment that only exists online. You can use it to buy products and services online from places that accept that as a form of payment – like they do with paypal, credit and debit cards, etc – a small but growing number of physical shops around the world are also beginning to accept bitcoin payment too.
Amongst its many features is the ability to conduct transactions relatively anonymously. This means that there is no bank or centralised organisation controlling it and therefore almost no ability to track who you have paid and what you bought.
Bitcoin is only one of many cryptocurrencies (Ethereum, LiteCoin, Bitcoin Cash, Zcash etc); but is probably the most well known. The main reason Bitcoin is attracting so much attention (apart from the price) is the anonymity and the price volatility – some times jumping and falling more than $5000 in a matter of hours !
For more info on buying / trading Bitcoin and other cryptocurrencies :