The hyphen removes any whitespace adjacent to the template construct. Often this isn't strictly necessary but it makes the rendered output more readable. Since YAML is whitespace-sensitive sometimes it is required.

Basic example:

environment:
{{ range .Values.xxx }}
  - { name: {{ . }}, value: "test" }
{{ end }}

Renders to:

environment:

  - { name: a, value: "test" }

  - { name: b, value: "test" }

On the range line, after the }} and before the -, there is a newline and two spaces; these are required for it to be valid YAML. Before both of the {{ there is an additional newline and this isn't required, so you can use {{- to suppress it.

environment:
{{- range .Values.xxx }}
  - { name: {{ . }}, value: "test" }
{{- end }}

environment:
  - { name: a, value: "test" }
  - { name: b, value: "test" }

This is also useful with functions like nindent that include their own newlines.

items: {{- .Values.xxx | toYaml | nindent 2 }}

items:
  - a
  - b

toYaml converts the list [a, b] to the YAML string - a\n- b; nindent 2 adds an initial newline, and two spaces at the start of each line \n - a\n - b; and then the - removes the extra space before the newline that's being emitted.

You can put the - inside the closing }} too, with the same effect, but that tends to eat the next line's indentation, so it tends to be a little less useful.

(Remember that helm template will render a chart to stdout without sending it off to the Kubernetes API server, so you can see what these things expand to.)