The expense ratio is a fund's operating cost expressed as an annual percentage of its assets: custody, audit, administration, and similar expenses. You never receive an invoice — it is deducted gradually from the fund's net asset value, so its effect shows up in the unit price itself.
For listed Saudi funds, Tadawul publishes the expense ratio and the management fee as two separate items; the total cost an investor actually bears is their sum. When comparing funds, compare that total, not one component.
The danger of fees is their silent compounding: a one-percentage-point difference per year sounds trivial, but over twenty years it removes a large slice of your final wealth, because whatever was deducted early would have compounded in your favour.
Fees are one of the very few things in investing that are known in advance: future returns are uncertain, but fees are deducted with certainty every year. That is why they carry heavy weight in any objective fund assessment.