As with all binary numbers, you can start from the right side. The furthest right place represents the 20 (=1) place. As you move left, the places represent 21, 22, 23, etc. So, as you go from the right to the left, you can find the decimal representation by multiplying the binary digit by the place it is in. Then, you should add up all of these numbers.
Using 10101011 starting from the furthest right, 120 = 1, 121 = 2, 022 = 0, 123 = 8, 024 = 0, 125 = 32, 026 = 0, 127 = 128.
Now, add these numbers together: 1+2+0+8+0+32+0+128 = 171.