You left out a line of pseudo-code:

It should be:

For i = 1 to N
W_t = {
    M_tⁱ  (when 0 ≤ t ≤ 15)
    ROTL¹ (Wt−3 ⊕ Wt−8 ⊕ Wt−14 ⊕ Wt−16) (when 16 ≤ t ≤ 79)
}


N is the number of blocks in the padded message, so the first line says to loop through all the blocks in the message (this explains where the ⁱ in the M_tⁱ comes from).

W_t is the t^th word of the message schedule, so when processing word #1, that would be W₁, when processing word #2, it would be W₂, and so on.

For each word of the message schedule, you perform one of two processes...

If you are processing any of the first 16 words in a message block (word #0 through word #15), then you simply set W_t to the value of that word:
M_tⁱ is just the t^th word of the i^th message block

If you are processing any of the remaining words in a message block (word #16 through word #79) then you set W_t to the result of the following function:
ROTL¹ (W_t−3 ⊕ W_t−8 ⊕ W_t−14 ⊕ W_t−16)

ROTL¹ means to perform a one bit circular left shift.

The thing that is being shifted is the result of:
W_t−3 ⊕ W_t−8 ⊕ W_t−14 ⊕ W_t−16

Remember that you are processing the t^th word and that this only happens when you are processing words 16 through 79, so for example:
If t is 17 then you perform:
W₁₇₋₃ ⊕ W₁₇₋₈ ⊕ W₁₇₋₁₄ ⊕ W₁₇₋₁₆
complete those subtractions and you find that you are performing:
W₁₄ ⊕ W₉ ⊕ W₃ ⊕ W₁
Since  is the symbol for the EXCLUSIVE OR (XOR) operation, you need to perform an XOR on those 4 words of the message schedule.

This set of instructions is performed for all 80 words in the message block (word #0 through word #79)