Assets are things the business owns that hold value over time: vehicles, machinery, equipment, property.
They are recorded in Accounting → Balance sheet, and they are what makes that statement mean something — a balance sheet with no assets on it describes a business that owns nothing.
Adding one
Record what it is, what it cost, and when it was bought.
The purchase date matters more than owners expect. It drives depreciation, it is what an insurer asks for after a loss, and it turns "the vans are getting old" into a replacement schedule you can budget for.
What counts
Above a value threshold your accountant sets, a purchase is an asset. Below it, it is an expense in the period you bought it — see tools and small equipment.
The threshold varies by region. What matters practically is that a large purchase does not hit the profit and loss all at once.
Assets and what the business is worth
Book value comes straight from what is recorded here — see business valuation. A business whose assets have never been recorded shows a book value that is wrong by the value of everything it owns.
Equipment you also track operationally
Laptops, phones and field equipment can be tracked in devices with who has them. Larger capital items belong here, on the balance sheet. Some businesses record both, which is fine — they answer different questions.