The custom report builder works like a drag-and-drop query tool: pick an object (contacts, deals, tickets), pick the properties and date range, choose a chart type, and save it. Cross-object reports, blending deal data with ticket data, say, are the more powerful version of this, and they're the part that's historically been gated behind higher tiers or the Reporting Add-on rather than included everywhere by default.
Dashboards are just collections of saved reports, arranged on one screen and shareable with a team or scheduled as an email digest on a recurring basis, which covers distribution inside HubSpot reasonably well. Standard, pre-built reports for things like deal velocity or email open rates come with every portal and don't require building anything from scratch.