The term brokerage account generally refers to an account that lets you buy/sell stocks and ETFs (exchange traded funds) which are mutual funds packaged up to trade like stocks. SPY is an example of an ETF. You could have a mutual fund account with Vanguard that is not a brokerage account--one that let you buy/sell mutual funds but not stocks.
Taxable is generally a term that refers to investment accounts that are not tax-advantaged in some way, generally retirement accounts like 401ks, traditional and Roth IRAs, 403bs, etc.--something that has a tax benefit, which generally also has some kinds of restrictions on how and when the money can be withdrawn.
If your employer matches some of your 401k contributions which is common, then generally you'd want to contribute up to the maximum amount that they match. What that means is that your employer contributes to your 401k account up to some level to match some or all of what you put in. If you don't invest up to that amount, you lose whatever they would have matched. For example, my employer matches 50% of up to 6% of my pay, so if I contribute 6% of my salary, they contribute 3% of my salary as well.
A common downside of some 401ks is that they may not be with a low cost provider like Vanguard, so the expense ratio for your fund choices may be a lot higher than a Vanguard or Fidelity IRA with index funds. Usually the match is worth enough to offset the downside at least up to the maximum level for the match. Beyond that level, it may be better to use the IRA, until you reach the IRA maximum limit.
Vanguard has at least some funds that don't require a big investment up front to start. For example, their STAR fund (VGSTX) has only a $1,000 minimum, while most other funds have a $3,000 minimum. You can switch funds whenever you want.