Fiat is the financial term that describes any traditional government-backed currency such as U.S. dollars, Euros, Mexican pesos, etc. When you're "depositing fiat" or "funding with fiat" it simply means transferring money from your regular bank account to your Etana Custody account for use on an exchange.
Digital assets is an all-encompassing term for today's digital currencies or coins. Think Bitcoin, Ethereum, and the hundreds of others available to buy and sell. When you are "holding digital assets" or want to "custody digital assets" it simply means you're storing your various types of coin or digital currencies in highly secure wallets that are a part of Etana's platform.