When you get in the shower, it's pretty noisy, and some of the sounds could have similar frequencies as a telephone ringing. So, in this condition, listening for the phone could make you think you're hearing it ringing when it isn't. You're a lot more likely to dash out of the shower by mistake when you're listening for the phone to ring than when you aren't. Get rid of thresholds, talk about detection. Sensitivity is the limit of your perceptual system.
Some differences are easy to detect, and some are quite difficult.
Bias is your willingness to say you detect something. The stronger the signal is relative to the noise, the easier it is to discriminate something from nothing, and the higher the sensitivity will be. Bias has to do with the perceiver's willingness to say that they see something. They have to place a criterion somewhere along the dimension of evidence for a signal. Placing the criterion in different locations changes the likelihood of these errors.