Strapping in circuits is figuratively tying a thing by strings so it stays in vicinity of voltage/current/circuit constants. In this case, the chip has pins that can be used to change operating modes by strapping high or low, called strapping pins. Similar to but slightly different from pull-up/pull-down.
Boot strapping in circuits usually refers to passive parts that provide power needed locally for switching parts.
Strapping pins internally have a pullup or pulldown resistor. At startup these control what state some part of the system is in, (like the example mentioned elsewhere, flash voltage). These are controllable by applying your own resistor at startup, like a normal i/o pin would be. The difference is that these pins are multipurpose. At power-on, the early bios reads the bootstrapping pins, sets its state, and then releases them to the actual OS, so that they can be used for other things (sometimes).
As useful as they are, imo they are a bit problematic, because of that mixed-use state. You have to be able to setup that pin for your hardware configuration, and then during operation use it for your needs.
Most of these also have an internal fuse you can burn to force the state permanently, which is where the "slightly different" comes in. The fuse sets the permanent internal pull state, so you dont have to deal with the setup phase, except, that you need to make sure anything you use that pin for in your application defaults to the state you want, as you can still externally modify that pin at power-on.
Boot strapping in circuits usually refers to passive parts that provide power needed locally for switching parts.