A column is a single property the database tracks on a record - a client's name, an invoice amount, a due date, a job's status. Every column has a defined type (text, number, date, a choice from options, a relation to another entity, a formula) and the same meaning across every record in that entity.
In more advanced database apps, access can be set at the level of individual columns, not just the whole table - a salesperson can see and edit a job record, while a column with the margin markup stays visible only to management. That's a finer level of control than a shared spreadsheet offers, where you either see the whole sheet or nothing at all.
Adding a new column (like "lead source" on a client) is a matter of configuration in a database app, not a developer task - unlike off-the-shelf systems, where a similar change often means filing a support ticket.
Related: What a CRM you can edit yourself looks like. In 20 minutes., Access rights and data visibility, A data model that grows with the team.