Electronic – What does “strobe” do for this shift register

3-state simply refers to the ability to disable the outputs, in this case using the OE pin for output enable. It really has nothing to do with the registers themselves, but only the output drivers. Doing it this way allows to set up a bunch of devices on a common bus; for example, a shift register like this could receive the current state of a NES controller, and be read out at a later time.

Page 3 of the datasheet shows the functional steps very clearly; there's one shifting chain, fed by SER and SRCLK (with output into QH' so you can chain it wider, as for a SNES controller), a second 8-bit register that is updated using RCLK, and buffers to read that out using OE. This setup allows one set of counting hardware to load new values without risking a partially shifted version of data on the outputs.

So yes, RCLK will copy the shift register contents into the parallel output register. From there the only readout available is using OE to enable the output pins QA through QH. That makes the 74*595 a serial to parallel shift register.

If I understand your question correctly, it is important to understand that the device works in a couple separate stages.

1. The parallel inputs. These have latches that must be signaled to read the inputs and internally hold their state. Once latched, the signal on the input pins can change but the internal signals will be stable. (RCK)
2. The next step is where these internal signals are copied into the shift register. (\$\overline{\small \text{SLOAD}}\$)
3. The last step is shifting out the bits in the shift register (SCK)

If I understand the truth table and timing diagram in the datasheet correctly then the correct order of steps is the following:

1. Initialize the signals as follows RCK = low; \$\overline{\small \text{SLOAD}}\$ = high; \$\overline{\small \text{OE}}\$ = low; SCK = low;
2. Apply parallel signals to the inputs A~H
3. Clock the parallel signals into the latches by driving RCK high. RCK triggers on a rising edge. Drive RCK low, its inactive state.
4. Copy the contents of the input latches into the shift register by driving \$\overline{\small \text{SLOAD}}\$ low. \$\overline{\small \text{SLOAD}}\$ is active low. Drive \$\overline{\small \text{SLOAD}}\$ high again, its inactive state.
5. Read pin Q_H on your MCU
6. Apply a clock pulse on SCK to shift the data out on pin Q_H. SCK triggers on a rising edge. Drive it high, then low.
7. Repeat last 2 steps for as many inputs as you are interested in.