sass代码中的">"大于和"+"加号代表什么意思？

Question

最近在看bootstrap的源码，看到这么一段。想问里面的 > 和 + 起什么作用？

.input-group {
  position: relative;
  display: flex;
  flex-wrap: wrap; // For form validation feedback
  align-items: stretch;
  width: 100%;

  > .form-control,
  > .form-control-plaintext,
  > .custom-select,
  > .custom-file {
    position: relative; // For focus state's z-index
    flex: 1 1 auto;
    // Add width 1% and flex-basis auto to ensure that button will not wrap out
    // the column. Applies to IE Edge+ and Firefox. Chrome does not require this.
    width: 1%;
    margin-bottom: 0;

    + .form-control,
    + .custom-select,
    + .custom-file {
      margin-left: -$input-border-width;
    }
  }

  // Bring the "active" form control to the top of surrounding elements
  > .form-control:focus,
  > .custom-select:focus,
  > .custom-file .custom-file-input:focus ~ .custom-file-label {
    z-index: 3;
  }

  // Bring the custom file input above the label
  > .custom-file .custom-file-input:focus {
    z-index: 4;
  }

  > .form-control,
  > .custom-select {
    &:not(:last-child) { @include border-right-radius(0); }
    &:not(:first-child) { @include border-left-radius(0); }
  }

  // Custom file inputs have more complex markup, thus requiring different
  // border-radius overrides.
  > .custom-file {
    display: flex;
    align-items: center;

    &:not(:last-child) .custom-file-label,
    &:not(:last-child) .custom-file-label::after { @include border-right-radius(0); }
    &:not(:first-child) .custom-file-label { @include border-left-radius(0); }
  }
}

Answer 1

代表子选择器，可以选择某个元素中所有的子元素， + 代表相邻元素选择器，选择紧挨着某个元素的元素。
如果有解释不清楚的地方请添加我的微信:tfzwgd，有关CSS的技术问题可以交流沟通。

Answer 2

一个是子选择器 一个是相邻选择器，百度比你在这提问更容易得到答案

Answer 3

你还是先学好css再考虑sass吧，别本末倒置了