A tender is a formal, competitive invitation from a buyer asking suppliers to submit priced offers for a defined piece of work, and also the offer a supplier submits in response. In public procurement, tenders are advertised openly so that any qualifying supplier can compete on equal terms.
The word is used both ways in everyday practice: a buyer 'publishes a tender' and a supplier 'submits a tender'. A published tender always sets out what is being bought, the conditions a supplier must meet to be eligible, the criteria by which offers will be scored, and the deadline for submission. Those four elements are what make it a tender rather than an informal request for a quote.
For suppliers, the practical significance is that the rules are published in advance and apply equally to everyone. You know before you start what the buyer wants, how they will score it, and when they need it — which is unusual in commercial sales and is the main reason public contracts are accessible to smaller firms. A tender is live and biddable only while its submission deadline is still in the future; after that it moves into evaluation and is eventually closed with an award notice.