Learn Dutch: Het saldo van mijn bankrekening is laag. - The balance of my bank account is low.

Saldo is a neuter noun, so it takes the definite article het. Many words borrowed from other languages that end in an '-o' are neuter in Dutch, such as het casino or het tempo.

How is the word bankrekening formed?

It is a compound noun made of bank (bank) and rekening (account or bill). In Dutch, you stick these words together into one single word without spaces. The gender of a compound word always follows its final part. Since it is de rekening, it becomes de bankrekening.

Why doesn't the adjective laag get an '-e' ending here?

In Dutch, adjectives usually get an -e ending when they are placed directly in front of the noun they describe. Because laag comes after the verb is in this sentence, it does not get an extra ending and stays in its basic dictionary form.

Could I combine the nouns into one big word instead of using van?

Yes, you absolutely can! Dutch is famous for stacking nouns. Het bankrekeningsaldo is a perfectly valid and common way to say 'the bank account balance'. Using van (of) to link them, as shown in this sentence, is just another natural way to express it.