t₁ = 1, the first term is 1 then
t₂ = 3, the second term is 3 then.
tₙ₊₂ = tₙ₊₁ + tₙ is another way of saying, the number after "n+1", which is "n+2", is going to be coming from the sum of the current term "n" plus the one right after "n+1" OR the next term is coming from the sum of the last two behind it.
to make it less jumbled, in short,
t₃ = 1+3 = 4
t₄ = 3 + 4 = 7
t₅ = 4 + 7 = 11.